With the development of optical disks in recent years, the variety of optical disks in use such as recording and reproducing optical disks and read-only memory (ROM) optical disks has been increasing. Under such circumstances, an optical head device that can reproduce information from these plural types of optical disks has been devised.
In the following, a conventional technology will be described with reference to FIG. 21. In FIG. 21, numeral 160 denotes an LD-PD (Laser Diode Photo Detector) unit that is arranged so as to emit light polarized in an x-axis direction. The LD-PD unit 160, which will be described more specifically later, has photodetectors for detecting light containing a signal and a semiconductor laser serving as a light source that are fixed in a certain positional relationship. Numeral 102 denotes a collimator lens. Numeral 180 denotes a polarization anisotropic hologram having a function of transmitting light polarized in a certain direction and diffracting light polarized in a direction perpendicular thereto, which is arranged so as to transmit light polarized in the x-axis direction. Numeral 115 denotes a ¼ wave plate, and numeral 103 denotes an objective lens. Numeral 106 denotes a holder for maintaining the positional relationship between the polarization anisotropic hologram 180, the ¼ wave plate 115 and the objective lens 103. Numeral 105 denotes an optical disk that is arranged so that its tangential direction corresponds to a y-axis direction. Numeral 112 denotes a driving member for driving the holder 106.
The following is an explanation of its operation. A linearly polarized light beam L0 emitted from a radiation light source in the LD-PD unit 160 is not diffracted by the polarization anisotropic hologram 180 because it is polarized in the x-axis direction, and then reaches the ¼ wave plate 115. This light beam further passes through the ¼ wave plate 115, becomes circularly polarized by an effect of the ¼ wave plate 115, enters the objective lens 103 and is converged on the optical disk 105 (outgoing path).
The light beam reflected by the optical disk 105 travels backward along the optical path, passes through the ¼ wave plate 115 again, becomes linearly polarized light beam whose polarization direction is perpendicular to that of the original light beam (the y-axis direction), and then enters the polarization anisotropic hologram 180. Returning +1-order diffraction light L1 and −1-order diffraction light L2 that are generated from the polarization anisotropic hologram 180 reach photodetectors arranged in the LD-PD unit 160 where a servo error signal and a recorded information signal are detected.
The following is a detailed description of how the signals are detected, with reference to FIGS. 22 and 23. FIG. 22 is a schematic view showing the polarization anisotropic hologram 180, and FIG. 23 is a schematic view showing the LD-PD unit 160.
As shown in FIG. 22, the polarization anisotropic hologram 180 is divided into large regions A, B, C and D by straight lines that pass through the center of the polarization anisotropic hologram 180 (the same as an optical axis) and are parallel to the x-axis and the y-axis, respectively. Furthermore, each of the regions is divided into small strap-like regions by a plurality of straight lines. Within one large region, hologram patterns with the same functions are formed in every other small strap-like regions. Hereinafter, the regions having the same hologram patterns are altogether referred to as one small region (region Ab, Af, Bb, Bf, Cb, Cf, Db, Df).
As shown in FIG. 23, the LD-PD unit 160 has a photodetector 191 and a photodetector 192, which are arranged on both sides of a light-emitting point (or a point equivalent to a light-emitting point) P. The photodetector 191 is divided into two regions across the y-axis direction, and each of them further is divided into two regions by a straight line that is parallel to the x-axis, thus forming regions FE1, FE2 and regions FE3, FE4. Also, the photodetector 192 is divided into four regions (regions TEa, TEb, TEc, TEd) by straight lines that are parallel to the x-axis and the y-axis, respectively.
The returning light that has entered the polarization anisotropic hologram 180 is converted into the returning +1-order diffraction light L1 and −1-order diffraction light L2 by a diffraction effect of the polarization anisotropic hologram 180.
As described above, the polarization anisotropic hologram 180 is divided into a plurality of regions, which are formed so as to diffract light in different directions and wavefronts. Each region of the polarization anisotropic hologram 180 is designed so as to function in the following manner when the smallest light spot is formed on a recording surface of the optical disk 105 (in a focused state).
The +1-order diffraction light L1 generated from the light that has entered each region of the polarization anisotropic hologram 180 shown in FIG. 22 reaches each position in the photodetector 191 shown in FIG. 23 as follows.
The light that has entered the region Ab reaches the position indicated by L1Ab in the photodetector 191 so as to converge on a back side (at the position with a smaller z coordinate) with respect to the photodetector 191. The light that has entered the region Af reaches the position indicated by L1Af in the photodetector 191 so as to converge on a front side (at the position with a larger z coordinate) with respect to the photodetector 191.
The light that has entered the region Bb reaches the position indicated by L1Bb in the photodetector 191 so as to converge on the back side (at the position with a smaller z coordinate) with respect to the photodetector 191. The light that has entered the region Bf reaches the position indicated by L1Bf in the photodetector 191 so as to converge on the front side (at the position with a larger z coordinate) with respect to the photodetector 191.
The light that has entered the region Cb reaches the position indicated by L1Cb in the photodetector 191 so as to converge on the back side (at the position with a smaller z coordinate) with respect to the photodetector 191. The light that has entered the region Cf reaches the position indicated by L1Cf in the photodetector 191 so as to converge on the front side (at the position with a larger z coordinate) with respect to the photodetector 191.
The light that has entered the region Db reaches the position indicated by L1Db in the photodetector 191 so as to converge on the back side (at the position with a smaller z coordinate) with respect to the photodetector 191. The light that has entered the region Df reaches the position indicated by L1Df in the photodetector 191 so as to converge on the front side (at the position with a larger z coordinate) with respect to the photodetector 191.
Next, the −1-order diffraction light L2 that is generated by the polarization anisotropic hologram 180 enters the photodetector 192 as follows.
The light that has entered the region Ab in FIG. 22 reaches the position indicated by L2Ab in FIG. 23. The light that has entered the region Af reaches the position indicated by L2Af.
The light that has entered the region Bb reaches the position indicated by L2Bb. The light that has entered the region Bf reaches the position indicated by L2Bf.
The light that has entered the region Cb reaches the position indicated by L2Cb. The light that has entered the region Cf reaches the position indicated by L2Cf.
The light that has entered the region Db reaches the position indicated by L2Db. The light that has entered the region Df reaches the position indicated by L2Df.
An optical head device with the above-described configuration can detect various signals in the following manner. A tracking error signal is detected by the photodetector 192. When detecting the tracking error signal, one of two methods is used suitably depending on the type of the optical disk 105. In other words, a push-pull method is used for an optical disk with a continuous groove (for example, a recording/reproducing optical disk), while a phase difference method is used for an optical disk with pit-shaped track information (for example, a ROM disk).
When a signal output from each region of the photodetector 192 is expressed by the name of this region, a tracking error signal TE according to the push-pull method can be obtained byTE=(TEa+TEb)−(TEc+TEd).  (1)The tracking error signal TE according to the phase difference method can be obtained by phase comparison of (TEa+TEc) and (TEb+TEd).
A focus error signal FE is detected by the photodetector 191. When a signal output from each region of the photodetector 191 is expressed by the name of this region, the focus error signal FE can be obtained byFE=(FE1+FE3)−(FE2+FE4).  (2)
A data signal S can be obtained by totaling signals from the photodetector 191 and the photodetector 192, i.e.,S=TEa+TEb+TEc+TEd+FE1+FE2+FE3+FE4.  (3)
In the optical head device with the conventional configuration described above, the data signal S has been detected by the sum of servo error signals (the focus error signal FE and the tracking error signal TE). Since the photodetector for detecting the servo error signals has to detect incident light in a defocused state, it has been necessary for the photodetector to have a large light-receiving area. An increase in the light-receiving area results in a larger capacitance of the photodetector. Consequently, the frequency characteristics of a detected signal deteriorate, leading to a problem that the data signal cannot be reproduced at a high speed.
Moreover, since the light-receiving area is large, the optical head device easily is affected by stray light. Thus, in a system with much stray light, for example, a system for reproducing information from an optical disk with many layers in which the information is recorded, a signal to noise ratio (S/N) deteriorates, causing a problem that an excellent reproduction signal cannot be obtained.